The present invention generally relates to a field emission display (FED) device and a method for fabricating the device and more particularly, relates to a field emission display device equipped with two cathodes, an anode and nanotube emitters as the electron emission source, and a method for fabricating the device by a thick film printing technique.
In recent years, flat panel display devices have been developed and widely used in electronic applications such as personal computers. One of the popularly used flat panel display device is an active matrix liquid crystal display which provides improved resolution. However, the liquid crystal display device has many inherent limitations that render it unsuitable for a number of applications. For instance, liquid crystal displays have numerous fabrication limitations including a slow deposition process for coating a glass panel with amorphous silicon, high manufacturing complexity and low yield for the fabrication process. Moreover, the liquid crystal display devices require a fluorescent back light which draws high power while most of the light generated is wasted. A liquid crystal display image is also difficult to see under bright light conditions or at wide viewing angles which further limit its use in many applications.
Other flat panel display devices have been developed in recent years to replace the liquid crystal display panels. One of such devices is a field emission display device that overcomes some of the limitations of LCD and provides significant advantages over the traditional LCD devices. For instance, the field emission display devices have higher contrast ratio, larger viewing angle, higher maximum brightness, lower power consumption and a wider operating temperature range when compared to a conventional thin film transistor (TFT) liquid crystal display panel.
A most drastic difference between a FED and a LCD is that, unlike the LCD, FED produces its own light source utilizing colored phosphors. The FEDs do not require complicated, power-consuming backlights and filters and as a result, almost all the light generated by a FED is visible to the user. Furthermore, the FEDs do not require large arrays of thin film transistors, and thus, a major source of high cost and yield problems for active matrix LCDs is eliminated.
In a FED, electrons are emitted from a cathode and impinge on phosphors coated on the back of a transparent cover plate to produce an image. Such a cathodoluminescent process is known as one of the most efficient methods for generating light. Contrary to a conventional CRT device, each pixel or emission unit in a FED has its own electron source, i.e., typically an array of emitting microtips. A voltage difference existed between a cathode and a gate electrode which extracts electrons from the cathode and accelerates them toward the phosphor coating. The emission current, and thus the display brightness, is strongly dependent on the work function of the emitting material. To achieve the necessary efficiency of a FED, the cleanliness and uniformity of the emitter source material are very important.
In order for the electron to travel in a FED, most FEDs are evacuated to a low pressure such as 10xe2x88x927 torr in order to provide a log mean free path for the emitted electrons and to prevent contamination and deterioration of the microtips. The resolution of the display can be improved by using a focus. grid to collimate electrons drawn from the microtips.
In the early development for field emission cathodes, a metal microtip emitter of molybdenum was utilized. In such a device, a silicon wafer is first oxidized to produce a thick silicon oxide layer and then a metallic gate layer is deposited on top of the oxide. The metallic gate layer is then patterned to form gate openings, while subsequent etching of the silicon oxide underneath the openings undercuts the gate and creates a well. A sacrificial material layer such as nickel is deposited to prevent deposition of nickel into the emitter well. Molybdenum is then deposited at normal incidence such that a cone with a sharp point grows inside the cavity until the opening closes thereabove. An emitter cone is left when the sacrificial layer of nickel is removed.
In an alternate design, silicon microtip emitters are produced by first conducting a thermal oxidation on silicon and then followed by patterning the oxide and selectively etching to form silicon tips. Further oxidation or etching protects the silicon and sharpens the point to provide a sacrificial layer. In another alternate design, the microtips are built onto a substrate of a desirable material such as glass, as an ideal substrate for large area flat panel displays. The microtips can be formed of conducting materials such as metals or doped semi-conducting materials. In this alternate design for a FED device, an interlayer that has controlled conductivity deposited between the cathode and the microtips is highly desirable. A proper resistivity of the interlayer enables the device to operate in a stable condition. In fabricating such FED devices, it is therefore desirable to deposit an amorphous silicon film which has electrical conductivity in an intermediate range between that of intrinsic amorphous silicon and n+ doped amorphous silicon. The conductivity of the nxe2x88x92 doped amorphous silicon can be controlled by adjusting the amount of phosphorous atoms contained in the film.
Generally, in the fabrication of a FED device, the device is contained in a cavity of very low pressure such that the emission of electrons is not impeded. For instance, a low pressure of 10xe2x88x927 torr is normally required. In order to prevent the collapse of two relatively large glass panels which form the FED device, spacers must be used to support and provide proper spacing between the two panels. For instance, in conventional FED devices, glass spheres or glass crosses have been used for maintaining such spacings in FED devices. Elongated spacers have also been used for such purpose.
Referring initially to FIG. 1A wherein an enlarged, cross-sectional view of a conventional field emission display device 10 is shown. The FED device 10 is formed by depositing a resistive layer 12 of typically an amorphous silicon base film on a glass substrate 14. An insulating layer 16 of a dielectric material and a metallic gate layer 18 are then deposited and formed together to provide metallic microtips 20 and a cathode structure 22 is covered by the resistive layer 12 and thus, a resistive but somewhat conductive amorphous silicon layer 12 underlies a highly insulating layer 16 which is formed of a dielectric material such as SiO2. It is important to be able to control the resistivity of the amorphous silicon layer 12 such that it is not overly resistive but yet, it will act as a limiting resistor to prevent excessive current flow if one of the microtips 20 shorts to the metal layer 18.
A completed FED structure 30 including anode 28 mounted on top of the structure 30 is shown in FIG. 1B. It is to be noted, for simplicity reasons, the cathode layer 22 and the resistive layer 12 are shown as a single layer 22 for the cathode. The microtips 20 are formed to emit electrons 26 from the tips of the microtips 20. The gate electrodes 18 are provided with a positive charge, while the anode 28 is provided with a higher positive charge. The anode 28 is formed by a glass plate 36 which is coated with phosphorous particles 32. An intermittent conductive layer of indium-tin-oxide (ITO) layer 34 may also be utilized to further improve the brightness of the phosphorous layer when bombarded by electrons 26. This is shown in a partial, enlarged cross-sectional view of FIG. 1C. The total thickness of the FED device is only about 2 mm, with vacuum pulled in between the lower glass plate 14 and the upper glass plate 36 sealed by sidewall panels 38 (shown in FIG. 1B).
The conventional FED devices formed by microtips shown in FIGS. 1A-1C produce a flat panel display device of improved quality when compared to liquid crystal display devices. However, a major disadvantage of the microtip FED device is the complicated processing steps that must be used to fabricate the device. For instance, the formation of the various layers in the device, and specifically, the formation of the microtips requires a thin film deposition technique utilizing a photolithographic method. As a result, numerous photomasking steps must be performed in order to define and fabricate the various structural features in the FED. The CVD deposition processes and the photolithographic processes involved greatly increase the manufacturing cost of a FED device.
In a co-pending application, Ser. No. 09/377,315, assigned to the common assignee of the present invention, a field emission display device and a method for fabricating such device of a triode structure using nanotube emitters as the electron emission sources were disclosed. In the triode structure FED device, the device is constructed by a first electrically insulating plate, a cathode formed on the first electrically insulating plate by a material that includes metal, a layer formed on the cathode of a high electrical resistivity material, a layer of nanotube emitters formed on the resistivity layer of a material of carbon, diamond or diamond-like carbon wherein the cathode, the resistivity layer and the nanotube emitter layer form an emitter stack insulated by an insulating rib section from adjacent emitter stacks, a dielectric material layer perpendicularly overlying a multiplicity of the emitter stacks, a gate electrode on top of the dielectric material layer, and an anode formed on a second electrically insulating plate overlying the gate electrode. The FED device proposed can be fabricated advantageously by a thick film printing technique at substantially lower fabrication cost and higher fabrication efficiency than the FEDs utilizing microtips. However, three separate electrodes are still required for the device, i.e., a cathode, a gate electrode and an anode which must be formed in separate process steps.
In another co-pending application, Ser. No. 09/396,536, assigned to the common assignee of the present invention, a field emission display device and a method for fabricating the diode structure device using nanotube emitters as the electron emission sources were disclosed. In the diode structure FED device, the device is constructed by a first glass plate that has a plurality of emitter stacks formed on a top surface, each of the emitter stacks is formed parallel to a transverse direction of the glass plate and includes a layer of electrically conductive material such as silver paste and a layer of nanotube emitter on top. The first glass plate has a plurality of rib sections formed of an insulating material in between the plurality of emitter stacks to provide electrical insulation. A second glass plate is positioned over and spaced-apart from the first glass plate with an inside surface coated with a layer of an electrically conductive material such as indium-tin-oxide. A multiplicity of fluorescent powder coating strips is then formed on the ITO layer each for emitting a red, green or blue light when activated by electrons emitted from the plurality of emitter stacks. The field emission display panel is assembled together by a number of side panels that joins the peripheries of the first and second glass plate together to form a vacuum-tight cavity therein. The FED device disclosed in this application can be fabricated with only two electrodes, i.e., the first electrode coated on the bottom glass plate and the second electrode coated on the top glass plate without the use of a gate electrode in between the two plates. In this configuration, the electron emitted from the nanotube emitters do not strike the phosphor coating layer on the top glass plate as hard as they would have when a gate electrode is utilized.
Various other fabrication and performance problems have been encountered in the above-described triode and diode structured FED""S. For instance, in the triode structure FED""s, the fabrication process is more complex and thus more difficult to achieve products of high reliability. The thickness of the insulating dielectric layer and the spacing between the electrodes must be formed with high precision in order to achieve reliability. In the diode structure FED""s, the distance between the cathode of phosphor coating and the anode of nanotube emitters must not be larger than 100 xcexcm. This limitation must be met in order to produce an operating field at 5 volts/xcexcm in a nanotube emitter type FED so that the driving voltage required stays in a reasonable range, i.e. smaller than 500 volts. The small distance allowed between the top plate (the anode) and the bottom plate (the cathode) causes other processing difficulties such as that of achieving a high vacuum in the cavity between the two plates, particularly when display panels of large dimensions are fabricated; and that of scattered electrons when only two electrodes are used resulting in reduced intensity and image quality.
It is therefore an object of the present invention to provide a FED panel that can be fabricated by a thick film printing technique which does not have the drawbacks or shortcomings of the conventional FED panel fabricated by thin film deposition processes.
It is another object of the present invention to provide a FED panel that can be fabricated by a thick film printing technique to form two cathodes and an anode in-between two panel substrates.
It is a further object of the present invention to provide a diode structure FED panel that can be fabricated by a thick film printing technique in forming a primary cathode, an auxiliary cathode and an anode for improved light intensity.
It is another further object of the present invention to provide a FED panel that can be fabricated by a screen printing technique and assembled without critical alignment requirements.
It is still another object of the present invention to provide a FED panel which utilizes two cathodes formed by a thick film printing technique and an anode of an ITO (indium-tin-oxide) electrode film formed by a thin film sputtering technique.
It is yet another object of the present invention to provide a FED panel wherein a dielectric layer of large thickness is used to insulate a primary cathode from an anode wherein the primary cathode is formed in two parallelly-positioned rows of electrodes formed of carbon nanotubes on each emitter stack.
It is still another further object of the present invention to provide a method for fabricating a FED panel by first using a thick film technique for forming a plurality of electron emitters in a nanotube material and then a thin film sputtering technique for forming an anode of ITO.
It is yet another further object of the present invention to provide a method for fabricating a FED panel by using a thick film printing technique for forming a plurality of nanotube emitters on top of a cathode layer by mixing nanometer dimensioned hollow fibers of carbon, diamond or diamond-like carbon in a polymeric-based binder.
In accordance with the present invention, a field emission display panel equipped with two cathodes, i.e. a primary cathode and an auxiliary cathode, and an anode and a method for fabricating the FED panel by a thick film printing technique are disclosed.
In a preferred embodiment, a field emission display panel that has two cathodes and an anode is provide which includes a first electrically insulating plate for use as a first panel substrate; a first plurality of emitter stacks formed substantially equally spaced from each other at a predetermined spacing in a longitudinal direction on the first electrically insulating plate, each of the emitter stacks is positioned parallel to a transverse direction of the first insulating plate and includes sequentially a layer of a dielectric material, a layer of a first electrically conductive material and a layer of nanotube emitters with the layer of dielectric material intimately contacting the first electrically insulating plate, the layer of nanotube emitters is formed in two rows spaced-apart and insulated from each other, a second plurality of nanotube emitters is formed spaced-apart and insulated from each other in each row; a second plurality of electrically conductive strips formed of a substantially transparent material coated on the first electrically insulating plate in-between the plurality of emitter stacks insulated from each other by the layer of dielectric material; a second plurality of fluorescent coating strips formed on the second plurality of electrically conductive strips each for emitting a red, green or blue light upon activation by electrons emitted from the second plurality of nanotube emitters on the first plurality of emitter stacks; a second electrically insulating plate positioned over and spaced-apart from the first electrically insulating plate for use as a second panel substrate, the second plate has on a surface facing the first plate a layer of the substantially transparent material coated; and a plurality of side panels joining peripheries of the first and second electrically insulating plates together forming a vacuum-tight cavity therein.
In the field emission display panel that is equipped with two cathodes and an anode, the second plurality of electrically conductive strips may further include a reflective coating layer coated in-between the second plurality of electrically conductive strips and the first electrically insulating plate. The reflective coating layer may be formed of a metal. The first and second electrically insulating plates may be formed of a ceramic material that is substantially transparent. The layer of the first electrically conductive material is a first cathode and the substantially transparent material coated on the second plate is a second cathode for the field emission display panel. The layer of the first electrically conductive material may be a silver paste. The second plurality of electrically conductive strips are an anode for the field emission display panel. The second plurality of electrically conductive strips may be formed of indium-tin-oxide. The layer of nanotube emitters may be formed of a mixture of nanometer dimensioned hollow tubes and a binder material. The layer of nanotube emitters may be formed of a mixture of nanometer dimensioned hollow tubes of carbon, diamond, or diamond-like carbon and a polymeric-based binder. Each of the second plurality of flourescent coating strips emits a light of red, green or blue that is different than the light emitted by its immediate adjacent strips when activated by electrons emitted from an adjacent pair of the second plurality of nanotube emitters. The layer of dielectric material in the first plurality of emitter stacks has a thickness between about 10 xcexcm and about 300 xcexcm.
The present invention is further directed to a method for fabricating a field emission display panel with two cathodes and an anode including the operating steps of first providing a first electrically insulating plate; forming a first plurality of electrically conductive strips of a substantially transparent material on the first electrically insulating plate, the first plurality of electrically conductive strips is an anode; forming a first plurality of emitter stacks on the first electrically insulating plate by a thick film printing technique parallel to a transverse direction of the first electrically insulating plate, each of the emitter stacks includes sequentially a layer of a dielectric material, a layer of a second electrically conductive material and a layer of nanotube emitters with the layer of dielectric material intimately contacting the first electrically insulating plate; forming a second plurality of nanotube emitters spaced-apart and insulated from each other in two parallelly situated and spaced-apart rows on top of each emitter stack from the layer of nanotube emitters, the second plurality of nanotube emitters is a first cathode; forming a second plurality of fluorescent coating strips on the first plurality of electrically conductive strips each for emitting a red, green or blue light when activated by electrons emitted from the second plurality of nanotube emitters on the first plurality of emitter stacks, positioning a second electrically insulating plate over and spaced-apart from the first electrically insulating plate, the second plate has on a surface facing the first plate a layer of the substantially transparent material coated forming a second cathode; and joining the first and second electrically insulating plates together by side panels and forming a vacuum-tight cavity therein.
The method for fabricating a field emission display panel with two cathodes and an anode may further include the step of providing the first and second electrically insulating plates in substantially transparent glass plates. The method may further include the step of printing the layer of a second electrically conductive material in a silver paste. The method may further include the step of printing the layer of nanotube emitters from a mixture of a binder and nanometer dimensioned hollow fibers selected from the group consisting of carbon fibers, diamond fibers and diamond-like carbon fibers. The method may further include the step of connecting a negative charge to each of the first and second cathodes and a positive charge to the anode. The first plurality of electrically conductive strips may be formed of indium-tin-oxide.
The method for fabricating a field emission display panel with two cathodes and an anode may further include the step of coating a reflective coating layer in-between the first plurality of electrically conductive strips and the first electrically insulating plate. The second multiplicity of fluorescent coating strips may be formed by a thick film printing technique. The second multiplicity of fluorescent coating strips may be formed such that each strip emits a red, green or blue light that is different than its immediate adjacent strips when activated by electrons emitted from an adjacent pair of the second plurality of nanotube emitters. The method may further include the step of coating the reflective coating layer in a metallic material, or in Cr or Al. The method may further include the step of forming the second plurality of fluorescent coating strips by a material that includes phosphor.